This invention relates to phase shift apparatus and particularly to variable phase shifters of a type in which plural phase shifted components of an input signal are formed and combined in proportions determined by a phase shift control signal to provide a phase shifted output signal.
Variable phase shift apparatus are useful, for example, as phase control elements in voltage controlled oscillators, tuning circuits, filters and in similar applications where controlled phase shift is needed. For example, in U.S. Pat. No. 4,020,500 entitled CONTROLLED OSCILLATOR which issued Apr. 26, 1977 to L. A. Harwood, there is described a voltage controlled crystal oscillator in which phase shift for controlling the oscillator frequency is provided by vector summation. Specifically, an oscillator signal is phase shifted by +90 degrees by means of a two-pole L-C low pass filter and combined with the original signal to produce a resultant vector lying between 0 degrees and a leading angle (e.g., +45 degrees) with the angle being controllable by varying the amplitude of the +90 degree (quadrature) signal. For providing lagging phase shifts (e.g., from 0 degrees to about xe2x88x9245 degrees) the polarity of the quadrature vector is inverted to be xe2x88x9290 degrees rather than +90 degrees and the inverted quadrature vector is added to the reference or input signal. Here the resultant vector lies between 0 degrees and a lagging angle between 0 degrees and about xe2x88x9245 degrees with the angle being controllable by varying the amplitude of the xe2x88x9290 degree vector.
A phase shift circuit, in an oscillator application, which avoids the need for quadrature phase shifting elements (e.g., 90 degree two-pole filtering) is described by Paul D. Filliman in his U.S. Pat. No. 4,797,634 entitled CONTROLLED OSCILLATOR which issued Jan. 10, 1989. Unlike the Harwood arrangement, the Filliman phase shifter combines three vectors at a time rather than two. The vector angles used are xe2x88x9245 degrees, 0 degrees and +135 degrees with respect to the input signal and the mixture of these three vectors provides all phase shifts, both positive (leading) and negative (lagging). Also, since the +135 degree vector is produced from the xe2x88x9245 degree vector by inversion, and since the xe2x88x9245 degree shift may be provided by a one-pole RC filter, the entire phase shift circuit may be readily constructed in integrated circuit form in applications such as providing phase control of color oscillators in television related products.
FIG. 1 herein presents a simplified block diagram of the Filliman oscillator. As shown, a phase shifter 10 is provided with two feedback paths comprising (i) a DC path (12) via a resistor (14) which regulates the DC operating point of the oscillator by means of negative feedback and (ii) an AC feedback path (16) through a resonator (18, e.g., a crystal) which provides positive feedback with a gain of unity for causing oscillations to occur.
Phase shifter 10, as illustrated in FIG. 1, is non-inverting whereas oscillator applications require inversion to provide the proper negative DC feedback for bias stabilization and positive AC feedback signal at the nominal resonator frequency, Fr, for causing oscillations to occur. The inversion of the feedback signal for oscillator applications is provided in FIG. 1 by an inverter 35 at the phase shifter output 22. Optionally, inversion may be applied at various places. For example, the inverter 35 may just as well be placed at the input 20 of the phase shifter 10 or within the phase shifter. Also, inversion may be provided by simply using the inverting output of a differential output amplifier in the phase shift network 10 or by using an inverting form of summing amplifier 24 at the phase shifter output. What is of importance to the present invention is not whether the shifter inverts or not, but how the phase shifting is accomplished as will be described.
Phase shifter 10 includes an input terminal 20 for receiving the oscillator input signal S1 to be phase shifted and an output terminal 22 for providing a phase shifted output signal S2. The input signal S1 separated into three xe2x80x9cintermediatexe2x80x9d signals or xe2x80x9cvectorxe2x80x9d components xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d which are summed via a summing circuit 24 to provide a phase controlled output signal S2 at the network 10 output terminal 22. Hereinafter, the terms xe2x80x9cintermediatexe2x80x9d signals or components may be used interchangeably with the term xe2x80x9cvectorxe2x80x9d signals or components.
The processing of the input signal S1 to form the intermediate signals or vector components xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d is provided by a constant gain amplifier 28, two variable gain amplifiers 30 and 32 (the latter of which is inverting) and a 45 degree phase lag network 26. Vector xe2x80x9cAxe2x80x9d in FIG. 1 has a phase angle of zero degrees with respect to the input signal S1 and is produced by applying the input signal to the summing circuit 24 via a non-inverting fixed gain amplifier 28.
Vector xe2x80x9cBxe2x80x9d has a phase angle of xe2x88x9245 degrees relative to the input signal S1 and is produced by phase shifting the input signal by 45 degrees (lagging) at the nominal frequency of the resonator 18 in the phase shift network 26 (e.g., a low pass filter). The phase shifted signal xe2x80x9cBxe2x80x9d is then applied to summing circuit 24 via a variable gain non-inverting amplifier 30.
Vector xe2x80x9cCxe2x80x9d has a phase angle of +135 degrees relative to the input signal S1 and is produced by inverting the output of the 45 degree phase shift network 26 with a controllable gain inverting amplifier 32 for application to summing circuit 24. A phase shift control signal 53 is applied directly to the gain control input of amplifier 30 and is inverted by means of an inverter 38 for application to the gain control input of inverting amplifier 32.
In operation, due to the action of the inverter 38, an increase in control signal S3 at terminal 22 will cause an increase in the amplitude of the vector xe2x80x9cBxe2x80x9d and concurrently will cause a decrease in the amplitude of the vector xe2x80x9cCxe2x80x9d. This will produce a lagging phase for the output signal S2 relative to the input signal S1. Conversely, a decrease in the control signal S3 will cause a decrease in the amplitude of vector xe2x80x9cBxe2x80x9d and an increase in the amplitude of vector xe2x80x9cCxe2x80x9d. This will produce a leading phase for the output signal S2 relative to the input signal S1. When changing phase angles, the amplitude of vector xe2x80x9cAxe2x80x9d is held constant. It is the variation of the amplitudes of vectors xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d which controls the net phase shift of the network and thus the frequency of the oscillator.
It has been found that under certain special circumstances (e.g., improper amplifier gains or gain variations) the Filliman type of oscillator, employing a three-vector phase shifter of the type described, may exhibit one, or more, of the following three operating difficulties: (i) stopped oscillations; (ii) overtone oscillations; and (iii) bias instability.
The problems noted above have been found to be traceable to the phase shift portion of the oscillator. It has also been found that the problems with the oscillator may be reduced by the expedient of restricting the phase range of the phase shifter. This, however, is not a completely satisfactory solution since it limits the oscillator frequency range and thus limits the range of useful applications for the oscillator.
Phase shift apparatus, embodying the invention, includes a source providing an input signal to be phase shifted and a combining circuit for combining first, second and third intermediate signals that are derived from the input signal, and have differing phase shifts, to form a phase shifted output signal. A first amplitude control circuit, responsive to a phase control signal supplied thereto, varies the amplitudes of the second and third intermediate signals in opposite directions for controlling the phase of the phase shifted output signal. Additionally, means are provided for reducing a tendency for variations in said phase shift control signal to alter the amplitude of the phase shifted output signal.